$\displaystyle \begin{array}{l}\text{Initial velocity }(u)=0m/s\\\text{Final velocity }(v)=20m/s\\\text{Time }(t)=5s\\\text{Acceleration }(a)=(v-u)/t=(20-0)/5=4m/{{s}^{2}}\\\text{Distance }({{s}_{1}})\text{ during acceleration}:\\{{s}_{1}}=ut+\frac{1}{2}a{{t}^{2}}+=0\times 5+\frac{1}{2}\times 4\times {{5}^{2}}=50cm\\\text{Deceleration phase:}\\\text{Initial velocity }(u)=20m/s\\\text{Final velocity }(v)=0m/s\\\text{Time }(t)=10s\\\text{Deceleration }(a)=(v-u)/t=(0-20)/10=-2m/{{s}^{2}}\\\text{Distance }({{s}_{2}})\text{during deceleration}:\\{{s}_{2}}=ut+\frac{1}{2}a{{t}^{2}}=20\times 10+\frac{1}{2}\times (-2)\times {{10}^{2}}=100m\\\end{array}$